The present invention generally relates to the field of noise blanking circuits in which noise impulses in a received signal are essentially eliminated by detecting these noise impulses and producing blanking pulses in response thereto which are used to prevent the passage of these noise impulses to a subsequent signal processing stage by blanking the received signal during the occurrence of the noise impulses. The present invention more particularly relates to such a noise blanker circuit used in a stereo radio receiver and such a noise blanker circuit which includes rate shut-off circuitry that prevents noise blanking action whenever highly repetitive noise impulses are detected.
The concept of eliminating noise from a received signal by producing corresponding blanking pulses which prevent the passage of noise impulses to a subsequent signal processing stage is well-known to those skilled in the art. Such noise blanking circuits are illustrated in U.S. Pat. Nos. 3,191,123, 3,284,714, and 3,699,457 all of which are assigned to MOTOROLA, Inc., the assignee of the present invention. Such noise blanking circuits generally detect noise impulses in a received signal by multiple differentiation of the signal. These detected noise impulses are then coupled to an amplifying device such that whenever the magnitude of a detected noise impulse exceeds the turn on threshold of the amplifying device, a corresponding blanking pulse will be produced. The blanking pulse is then coupled to a gate circuit which receives the input signal containing noise and passes this signal whenever a noise blanking pulse is not being produced. Typically a delay circuit is provided for delaying the time at which the noise containing input signal is received at the gate means such that a noise impulse will be received at the gate means at the same time that its corresponding noise blanking pulse is received by the gate means. In this manner the blanking pulse will prevent the gate means from passing the noise impulse.
Some noise blanking systems include rate shut-off circuitry which senses when the received signal contains noise impulses that occur at a rate in excess of a maximum predetermined rate. Under such conditions, the rate shut-off circuitry will prevent or inhibit the noise blanking circuit from producing noise blanking pulses. This can be desirable since otherwise the noise blanking pulses could result in totally blanking the received input signal such that no signal would be passed by the gate means whenever noise impulses in the received signal occurred at a rate exceeding the maximum predetermined rate. Thus without any rate shut-off circuitry it would be possible for the operator of a radio receiver to be totally unaware of a received radio transmission in which the received signal had a very high noise impulse repetition rate.
Some prior art noise blanking systems do sense the magnitude of the signal being received and use this information to decrease the threshold sensitivity of the noise blanker such that the noise blanker is prevented from generating blanking pulses whenever the magnitude of the detected noise impulses is relatively low compared to the magnitude of the input information signal which contains these noise impulses. In addition, some prior art noise blankers have been provided with a manual actuator for turning the noise blanking circuitry on and off.
Basically, the above mentioned prior art noise blanker systems have not provided any way to selectively alter the operation of the noise blanker circuit to obtain the most efficient mode of noise blanking for any particular type of noise impulse interference. For example, when the operator of a radio receiver is listening to a broadcasted musical composition and random small magnitude noise impulses are present, the most efficient mode of noise blanking to produce a noise free signal may correspond to adjusting the threshold level of the noise blanker such that it will blank even the smallest magnitude noise impulses, whereas if a great number of these noise impulses are present the most efficient mode of noise blanking to obtain a pleasing sound for the operator of the radio receiver may be obtained by having a slightly higher noise blanker threshold. In addition, noise blanker rate shut-off circuits are generally designed such that substantially no blanking will occur or only blanking of random large magnitude noise impulses will occur whenever the rate of repetition of noise impulses exceeds a maximum predetermined rate. This maximum rate is a function of the circuit design of the noise rate shut-off circuitry and represents an estimate as to when the operation of the noise blanker circuit should be shut off or altered in response to the occurrence of noise impulses which occur at a highly repetitive rate. For many types of information signals with noise impulse disturbances, this estimated maximum repetition rate at which the noise blanker circuit will be de-activated by the rate shut-off circuitry will not produce the most efficient mode of noise blanking. However, prior art noise blanker circuits have ignored this problem and have provided no way of altering the operation of the rate shut-off circuitry such that noise blanking of highly repetitive noise can be selectively implemented to obtain the most desirable mode of noise blanking operation.